1. Field of the Invention
This invention is generally directed to the removal of cationic impurities from inorganic solutions and more specifically to the removal of metals such as magnesium, calcium, potassium, iron, sodium, vanadium, aluminum and environmentally sensitive metals, such as cadmium, from wet process phosphoric acid using an ammonium ion or hydrogen ion loaded ion exchange resin which is regenerated utilizing a saturated solution of ammonium sulfate. The metals stripped from the ion exchange resin are removed from solution by precipitation and filtered thereby conditioning the ammonium sulfate solution for further use. The process is particularly suitable for removing significant amounts of magnesium and cadmium from wet process phosphoric acid solutions as well as other trace metals to thereby obtain a relatively pure phosphoric acid which may be processed into fertilizer products. The process, however, may be utilized in other acid processing such as in the production of boric acid.
2. Description of the Related Art
When phosphate rock is reacted with phosphoric acid and sulfuric acid to produce phosphoric acid, known as the wet process, many impurities that are in the rock are dissolved in the acid. These impurities are principally iron, aluminum, fluorine, magnesium, calcium, sodium, potassium and cadmium. Such impurities create problems in processing the acid and in converting it into fertilizer products. The impurities react to form sludge when the acid is concentrated and thereby increase the acid viscosity and also cause grade problems in the production of ammoniated phosphate fertilizers, such as monoammonium (MAP) and diammonium (DAP) phosphates. Low impurity phosphate reserves are being depleted on a worldwise basis with much of the remaining reserves containing significantly higher levels of magnesium and other impurities. Current ore beneficiation methods are not effective or economical in reducing the magnesium from the ore prior to producing acid. Existing ion exchange processes to remove these impurities from the phophoric acid are limited in the levels of magnesium that can be removed and often require large expenditures for chemicals used in resin regeneration. Also, the disposal of waste impurities and contaminated regeneration solutions requires additional material handling and increased production costs as such byproducts must be disposed of in such a manner as to prevent any damage to the environment.
One ion exchange process for removing magnesium from wet process phosphoric acid is known as the Swenson FBCSP purification system. This process uses a strong acid cation exchange resin in the hydrogen form. Regeneration of the resin is accomplished using a solution of sulfuric acid. The magnesium is precipitated from the spent regeneration sulfuric acid by raising the sulfuric acid concentration to 75% by the addition of 98% H.sub.2 SO.sub.4. After filtration, the 75% sulfuric acid solution is sent to the phosphoric acid attack system for acidulation of the phosphate rock. Because this regeneration system is directly tied to the reaction system, the amount of H.sub.2 SO.sub.4 available for regeneration is limited, which thus limits the amount of magnesium which can be removed from the phosphoric acid. In addition, the water added to the acidulation system by the dilute (75%) sulfuric acid versus the normal 98% sulfuric acid causes serious water balance problems in the circuit. Higher levels of magnesium in the phosphoric acid require greater quantities of regeneration sulfuric acid than that which can be used in the reaction system. This results in a limitation of the amount of magnesium which can be removed from the phosphoric acid without the purchase and subsequent handling and disposal of large amounts of sulfuric acid.